This mechanical separation may be carried out along various types of separation interfaces.
A first type of separation interface is a bonding interface, for example, a direct bonding (i.e., molecular bonding) interface.
The expression “direct bonding” is understood to mean bonding via intimate contact between two surfaces involving adhesive forces, mainly van der Waals forces, and not using any adhesive layers.
A structure able to be separated along a bonding interface, for example, a direct bonding interface, is known to those skilled in the art as a “debondable” structure.
Without wishing to be restrictive, it is however possible to state that this type of debondable structure may be used mainly in four different applications:                a) bonding of a mechanical stiffener: it may be desirable to bond a mechanical stiffener to a substrate or a fragile thin layer in order to prevent it from becoming damaged or breaking during particular fabrication steps, then to be able to remove this mechanical stiffener when its presence is no longer necessary;        b) correction of poor bonding: the debonding allows two substrates that have not been correctly bonded to be debonded then re-bonded after cleaning, in order to improve the profitability of a fabrication process and prevent, for example, poorly bonded substrates from being scrapped;        c) temporary protection: in certain steps of storing or transporting substrates, especially in boxes made of plastic, it may be useful to temporarily protect their surfaces, especially those intended to be used subsequently for the fabrication of electronic components, in order to prevent any risk of contamination. A simple solution comprises bonding two substrates so that their sides to be protected are respectively bonded to each other, then to debond these two substrates when they come to be used; and        d) double transfer of a layer: comprises producing a reversible bonding interface between an active layer and a first carrier substrate (optionally made of an expensive material), then transferring this active layer to a second final substrate by debonding the reversible bonding interface.        
A second type of separation interface is what is referred to as a “weakened” interface, which, for example, designates a zone obtained by implanting atomic species. Separation through such an interface is used in processes for transferring a layer from a first substrate to a second, an example being the process known by the trade name “SMART CUT®.”
The weakened interface may also be a porous zone.
A third type of separation interface is the interface between a first and second material, which may have been joined to each other by adding the second to the first using a deposition technique, for example, CVD or an epitaxial deposition technique.
Whatever the envisaged application is, it is necessary to carry out this separation without damaging, scratching or contaminating the surface of the two substrates located on either side of the separating interface, and without breaking these two substrates. Depending on the application in question, these two “substrates to be separated” may be two layers of a given substrate or two separate substrates.
Furthermore, the larger the size of the two substrates of the structure to be separated or the higher their separation energy (i.e., the energy required to separate them), the more difficult it is to separate them, especially without damage.
It is moreover known, from the research work of Maszara on the subject of the energy required to separate two substrates, that it is possible to measure the bonding energy between these two substrates by inserting a thin blade between them, level with their bonding interface (see the article by W.P. Maszara, G. Goetz, A. Caviglia and J. B. McKitterick, J. Appl Phys. (1988) 64:4943).
On this subject, the reader may refer to FIG. 1, which is a schematic showing the insertion of a thin blade of thickness d between two substrates, this having the effect of producing a gap between the two substrates over a length L.
Maszara established the following relationship:
  L  =                    3        ⁢                                  ⁢                  Et          3                ⁢                  d          2                            32        ⁢        γ              4  in which d represents the thickness of the blade inserted between the two bonded substrates, t represents the thickness of each of the two bonded substrates, E represents Young's modulus along the axis of the debonding, γ represents bonding energy and L represents the length of the gap between the two substrates at equilibrium.
In the above formula, it is assumed that the two substrates are of identical size.
By virtue of the aforementioned relationship, it is possible, by measuring L, to determine bonding energy γ.
However, it will be noted that in this article, the blade-induced debonding is not employed as an end in itself, but simply to measure bonding energy. Thus, the blade used is thin (between 0.05 mm and 0.5 mm in thickness) and risks scratching the bonded sides of the two substrates. This was of no importance in the trials of Maszara since the aim was to measure a bonding energy. However, such a technique is not envisaged when it is desired to reuse the substrates.
Insofar as the substrates or layers to be separated are stiff enough to be separated with a blade, it is possible to separate them by parting them sufficiently from each other, at their chamfered edge, this having the effect of creating a separating wave. The latter propagates from the point on the edge of the substrates at which it was initiated, toward the rest of the area of these substrates.
However, it will be noted that although the blade is inserted between the chamfers of the two substrates, the actual separation in fact occurs at the interface of separation where the bonding energy is lowest, i.e., where the amount of energy required to cause the separation is lowest.
A device for cleaving a structure comprising two wafers, bonded to each other via a bonding interface, and one of which contains a weakened zone, is already known from U.S. Pat. No. 7,406,994.
This cleaving device comprises a frame comprising two stationary parts and a movable central part, a cleaving blade that is translationally movable in the plane of the bonding interface and a stationary positioning element that is arranged on the frame opposite the blade. The structure to be cut is placed horizontally on the frame and held on a portion of its edge by the positioning element.
Once the blade has advanced sufficiently to be inserted into the groove that exists between the two wafers at the bonding interface, the movable central part of the frame is withdrawn so that the two wafers can part from each other. The separation occurs along the weakened zone.
This device has the drawback that, once the two wafers have been separated, they are not supported and risk being damaged or bonding to each other again.
The cleaving blade used has a concave circularly arcuate shape, the leading edge of which has a triangular cross-section of constant angle (i.e., it has one edge and two beveled sides).
Furthermore, as specified in column 7, lines 22 and 23, and in column 6, lines 23 and 24 of U.S. Pat. No. 7,406,994, the thickness of the blade is a few millimeters, preferably about 5 mm.
However, U.S. Pat. No. 7,406,994 provides no teaching on the possibility of separating substrates of large diameter or having a large bonding energy, for example, at least 1 J/m2, even 1.5 J/m2. Such a thin blade does not allow this to be done because it does not allow the substrates to be sufficiently parted.
Furthermore, the risk of scratching the surfaces with such a blade is not negligible.
In addition, the angle of the leading edge of the blade is large (indicated as being about 60°). Thus, when this blade is inserted level with the respective chamfers of the two substrates to be detached, the resultant of the forces exerted on the two substrates is directed mainly in a direction parallel to the bonding interface and not perpendicular to the latter. It is, therefore, difficult to part the two substrates from each other.